The change log indicates that the Windows binary build has been updated to use Python 2.7 (as of 0.7.30), but it's a little unclear (or I just missed it) if the Linux and OS X builds are now on 2.7, as well. (and if so... at what point) .

The reason I ask is that I have plugins that import compiled modules which may squawk about 'magic numbers' when importing modules from an older python build into a newer one.

I don't foresee much trouble handling the situation programmatically, but I need to verify if all three OS builds were switched to Python 2.7 at the same time... or if not, when.

No, the changelog mentions when each one did. For linux IIRC the entry is "all new linux binary with updated libraries" and for OS X it is new OS X binary build which only runs on os x leopard and higher.